1. Field of the Invention
The present invention relates to an antenna sharing device provided with piezoelectric resonator filters using a piezoelectric thin film resonators (film bulk acoustic wave resonators), and to a portable telephone incorporating the antenna sharing device.
2. Related Art
The film bulk acoustic wave resonator (FBAR) is an element formed in which a thin film piezoelectric body made of aluminum nitride (AlN), zinc oxide (ZnO), or the like is sandwiched by a pair of electrodes, and the element is formed on an air gap formed on a substrate. The film bulk acoustic wave resonator has approximately the same electric characteristics as a surface acoustic wave (SAW) resonator, and hence, it is possible to form a filter and an antenna sharing device for a high frequency circuit used in a portable telephone or the like by combining the film bulk acoustic wave resonators.
The antenna sharing device is a device which branches and couples signals in order to perform transmission and reception in different frequency bands by means of one antenna, and is referred to as a branching filter and a duplexer. The antenna sharing devices are widely used as a fundamental configuration in which one of input/output terminal pair is connected to a common signal terminal pair, and a phase shifter and a matching circuit are inserted in the common signal terminal pair or between the filter and the common signal terminal pair (see Japanese Patent Laid-Open No. 10-313229, Japanese Patent Laid-Open No. 2001-24476, Japanese Patent No. 3487692).
As the filter in the antenna sharing device, a band pass filter formed by using a 1-terminal pair resonator consisting of a surface acoustic wave resonator or a film bulk acoustic wave resonator as a serial resonator and a parallel resonator, and by connecting the resonators in a ladder shape, or the like, has been used.
The phase shifter and the matching circuit in the antenna sharing device are provided in order that a signal inputted from the common signal terminal pair into one filter or a signal outputted from the one filter to the common signal terminal pair is prevented from being inputted into the other filter. Specifically, the input impedance of the other filter is made very larger than that of the signal in the pass band of the one filter by the phase shifter and the matching circuit, to prevent the signal from being inputted to the other filter.
An inductor referred to as an extension coil or the like is connected to the parallel resonator constituting the filter in the antenna sharing device. As the stopband of the one filter, an attenuation pole is formed in frequencies corresponding to the pass band of the other filter to provide a large attenuation quantity for the one filter, thereby suppressing the leakage of transmission and reception signals. The phase shifter, the matching circuit, and the extension coil, as described above, can be produced in a package of the antenna sharing device, and on a ceramic substrate, a resin substrate or the like on which the filter is mounted.
With the above described constitution of the antenna sharing device, it is possible to produce an antenna sharing device which is capable of passing a signal at a low loss in the pass band of one filter, preventing the signal from being inputted into the other filter, greatly attenuating a signal in the pass band of the other filter, and having an increased isolation quantity between the input terminal pair and the output terminal pair which are provided for the two filters.
As a technique relating to the above described antenna sharing device, a multi-terminal pair SAW filter and a method of using the filter have been proposed in Japanese Patent Laid-open No. 10-313229. In the filter disclosed in the patent document, an antenna sharing device has two ladder type filters with different pass bands formed by using a plurality of surface acoustic wave resonators, and one of the input terminal pair and the output terminal pair of the respective ladder type filters is connected to a common terminal, and further, a matching inductor is connected to the common signal terminal pair.
As another technique, for example, a transmission/reception switching device has been proposed in Japanese Patent Laid-open No. 2001-24476. In this transmission/reception switching device, an antenna sharing device has two ladder type filters with different pass bands formed by using a plurality of film bulk acoustic wave resonators, and one of the input terminal pair and the output terminal pair of the respective ladder type filters is connected to a common signal terminal pair, and further, a phase shifter is inserted between one of the ladder type filters and the common terminal. Here, extension coils are connected to the parallel resonators.
As another technique, for example, a branching filter has been proposed in the patent document 3. In this branching filter, an antenna sharing device has two ladder type filters with different pass bands formed by using a plurality of surface acoustic wave resonators, and one of the input terminal pair and the output terminal pair of the respective ladder type filters are connected to a common signal terminal pair, and further, a matching circuit having inductors and capacitors is inserted between the respective ladder type filters and the common signal terminal pair.
In recent years, in mobile communication devices such as a portable telephone and an automobile telephone, in order to meet the demand for miniaturization of the devices, the antenna sharing device is desired to be further miniaturized. However, in the case where element intervals between the matching inductor, the phase shifter, the matching circuit, which are provided for the common terminals, and the extension coil or the like are reduced in accordance with the miniaturization, there is a possibility that the crosstalk of a signal is generated in the above described prior art, and thereby the attenuation quantity of the attenuation pole formed in the block band and the isolation quantity between the filters are reduced.